1. Field of the Invention
This invention relates to read-only memory devices in general, and more particularly, to metal oxide silicon read-only memories.
2. Description of the Prior Art
The earliest semiconductor memories used diode matrices. The presence or absence of a diode indicated data in each of the memory cell locations. These diode memories were referred to as read-only memories since these matrices could not store additional information without being physically altered.
Later read-only memories were fabricated with metal oxide silicon field effect transistors in arrays. The actual information was programmed into the array by actually fabricating the gates of the transistors. Therefore when the memory was read, if a gate was present for the addressed location, a voltage output would result. If no gate had been fabricated, no voltage output would result. One such read-only memory is described in U.S. Pat. No. 3,934,233 entitled, "Read-Only Memory for Electronic Calculator", by Roger J. Fisher and Gerald D. Rodgers and assigned to Texas Instruments. This type of read-only memory (ROM) includes a virtual ground architecture with a precharge system to conserve power. In a precharge system, the precharge metal lines which act as gates to the memory transistor devices become floating in curret architectures during the read cycle. These floating metal lines can be coupled to nondischarging diffusions as other diffusions underneath them. These diffusions partially discharge when they should have remained precharged. This erroneous discharge can result in incorrect data being read from the ROM. To decrease the effect of coupling, designers usually precharge several of the diffusions to the supply voltage. Ideally, the diffusions of the ROM can only be precharged to a value just below the flip point of the sense amplifier that is connected to the output line of the ROM device. By increasing the amount of voltage necessary to flip the sense amplifier, the integrity of the data is maintained. However, the increased voltage swing requires a longer time to discharge. Because of that fact, ROMs contained in present integrated circuits limit the speed at which these integrated circuits may operate. Secondly, since the diffusions are precharged to a higher voltage level, the charge coupled into metal lines may be injected into the substrate resulting in the forward-biasing of the diffusions that are attached to those metal lines. This injection charge may be collected by neighboring precharged diffusions causing an erroneous discharge.
It is an object of the invention to reduce the coupling between the floating metal gate lines and the diffusions in the substrate.